1. Field of the Invention
This invention relates to printed circuit boards (PCB's). More particularly, it relates to printed circuit boards having both plated through-holes and solder pads for surface mount electronic components.
2. Description of the Problem and the Related Art
In integrated structures such as those associated with semiconductor circuits commonly used in digital computers, the first level of packaging consists of a number of switching circuits fabricated on a single part called a chip. The chip is mounted in a structure called a module, which provides environmental protection for the chip and permits electrical interconnection to be made to the next level of package. Modules are normally provided in standard configurations to allow for the design of general-purpose manufacturing equipment and higher-level packages. One particularly common package is the dual-in-line pin ("DIP" or "d-i-p") configuration in which a generally rectangular plastic block containing the semiconductor chip is equipped with two rows of equally-spaced pins which exit the encapsulating plastic block on its opposing longer sides and are provided with a right-angle bend which directs the leads in the downward direction. The Texas Instruments 7400 series of integrated circuits typically have d-i-p leads spaced 0.100 inch apart. However, later generations of d-i-p chips have smaller lead spacings, now commonly on the order of 0.050 inch.
Signal wires (traces) on printed circuit boards carry signals between modules and between modules and input/output connections to the circuit board. On computer circuit boards the terminals that carry the logic signals to the modules and to the input/output terminals are called logic service terminals (LST's), those that supply power to the various modules are called power service terminals (PST's), and connections between wiring layers in the board are called vias or via service terminals (VST's). LST's and PST's may be plated through-holes which extend the full thickness of the board and open to solder pads on both surfaces of the board. Electrical or electronic components ("modules") designed to mount to PCBs by means of plated through-holes typically have depending metal leads which are spaced apart and sized to fit into corresponding plated through-holes and extend a small distance (approximately 0.1 inch) beyond the undersurface of the PCB (often called the "solder side" of the PCB).
An alternative to through-hole techniques for mounting components on PCBs is surface mount technology (SMT) wherein the leads of electrical components are soldered to metal pads plated on the surface of a printed wiring board. In this technique, a solder paste comprising solder powder and a solder flux or vehicle system is screened onto the PCB through a stencil or screen to cover discreet solder pads. Powder size and shape affect the solder paste by determining the applications method, printability, and tendency toward solder balling. Powder in the 40 to 75 micron range is primarily used for stencil applications. Powder in the 20 to 45 micron range is primarily used for syringe, screen printing and fine pitch stencil applications. Following application of the solder paste, electrical components are precisely placed on the PCB such that their coplanar leads contact corresponding pads on the circuit board which are coated with a layer of solder paste. The solder paste typically has sufficient adhesive strength to hold the components in position until the solder is melted. However, if additional mechanical security is required (for instance if the PCB will be inverted and heated with components hanging on its underside) a separate adhesive may be employed to further secure the components to the PCB.
After application of the solder paste and placement of the electrical components, the entire PCB assembly is heated in a reflow oven to melt the solder in the solder paste thereby forming solder joints which permanently affix and electrically connect the components to the PCB. The assembly is then washed to remove the flux residue and tested.
Although use of SMT components generally offers higher circuit densities inasmuch as the spacing between leads of SMT components (the lead "pitch") can be significantly reduced from that of equivalent through-hole devices, at the present state of the art, SMT components generally have significantly less mechanical strength in terms of their physical connection to the PCB than devices mounted by means of through-holes. For most electronic components, this strength difference is inconsequential and if such a problem exists, the use of an adhesive to secure the package to the surface of the PCB will usually suffice. However, for electrical components such as cable connectors and chip sockets, mechanical strength is of great importance since the device must not only provide electrical connection, it must also withstand the mechanical loads imposed by connecting and releasing cables, plug-in components, and the like. At present, there are few if any surface mount components which can substitute for through-hole devices of this sort. Accordingly, many printed circuit boards are equipped with both solder pads for SMT components and plated through-holes for devices such as sockets and cable connectors. Conventionally, through-holes devices are connected to the PCB by machine wave soldering--i.e., the solder side of the circuit board is passed across a wave of molten solder. If two clean metal surfaces are held together and dipped into molten solder, the solder will wet the metal and climb up to fill the gaps between the adjacent surfaces. This phenomenon is the result of capillary action. If the metal surfaces are not clean, the solder cannot wet the surfaces to be soldered, and the solder will not fill the joint. Capillary action causes the molten solder to fill the annular space between the component lead and the walls of the plated through-hole. Upon cooling and solidifying, the solder forms a fillet around the lead and the ends of the plated through-hole.
As is generally known in the art, it is possible to mount through-hole components to PCBs using screened solder paste. In this technique, solder paste is screen printed onto the through-holes in the PCB which are designed to receive the electrical leads of through-hole components. The components are subsequently inserted (i.e., the leads of the components are pushed through the solder paste and into the through-holes, carrying some of the solder paste with them) and the assembly is then heated in a reflow oven to melt the solder which forms fillets around the leads of the through-hole components.
However, a problem arises when this technique attempted on components have leads spaced less than about 0.075 inch apart. Apparently, as the leads of the component are pushed into the plated through-hole covered with solder paste, some of the solder paste is displaced laterally, causing a solder bridge to occur between adjacent leads (or plated through-holes) when the PCB is heated in the reflow oven.
The present invention solves this problem.